LCDs are commonly used as displays for compact electronic apparatuses. This is because they not only provide good quality images with little power consumption, but also they are very thin. The liquid crystal material in an LCD device does not emit any light itself. The liquid crystal material has to be lit by a light source so as to clearly and sharply display text and images. Thus, a backlight module is generally needed for an LCD device.
Referring to FIG. 13, a conventional backlight module 10 includes a light guide plate 110, a plastic frame 120 accommodating the light guide plate 110, a circuit board 130, and a plurality of light emitting diodes (LEDs) 140 connected to the circuit board 130.
The light guide plate 110 includes a light emitting surface 111, a light incident surface 112 adjacent to the light emitting surface 111, and two side surfaces 114 both adjacent to the light incident surface 112. The side surfaces 114 are at opposite lateral sides of the light guide plate 110. A pair of ears 115 outwardly extends from each side surface 114. The plastic frame 120 includes a plurality of notches 122 corresponding to the ears 115 of the light guide plate 110, and a plurality of cutouts 124 for accommodating the LEDs 140.
When the backlight module 10 is assembled, the light guide plate 110 is received in the plastic frame 120, with the ears 115 engaging in the notches 122. The LEDs 140 are respectively received in the cutouts 124, and are thus disposed adjacent to the light incident surface 112 of the light guide plate 110.
Due to imprecision in the manufacturing process, the sizes of the cutouts 124 may not accurately match the sizes of the LEDs 140. That is, after the backlight module 10 has been assembled, small gaps are liable to exist between the LEDs 140 and the light incident surface 112 of the light guide plate 110. Furthermore, in the process of assembly or when the backlight module 10 is in use, shaking or vibration of the backlight module 10 is liable to enlarge the gaps. These gaps may cause a reduction in the emitting luminance of the light guide plate 110.
FIG. 14 is a graph showing a relationship between the above-mentioned gaps and an efficiency of the emitting luminance. The X-coordinate represents the distance between each of the LEDs 140 and the light incident surface 112 of the light guide plate 110, in millimeters (mm). The Y-coordinate represents the relative emitting luminance of the light guide plate 110. It can be seen that the greater the value of X, the lower the value of Y. That is, the greater the gaps, the lower the value of the relative emitting luminance.
What is needed, therefore, is a backlight module that can overcome the above-described deficiencies. What is also needed is an LCD device employing such a backlight module.